The present invention relates to a process for monohydroxylation of phenolic compounds for the purpose of obtaining dihydroxylated aromatic compounds. The invention relates, more particularly, to a process for monohydroxylation of phenols and phenol ethers using hydrogen peroxide.
Several processes for hydroxylation of phenols are described in the prior art.
For example, the patent FR-A 2 071 464 which relates to a very important industrial process for hydroxylation of phenols and phenol ethers.
This process involves hydroxylation using oxygenated water in the presence of a strong acid. The most widely used of these strong acids are sulphuric acid, paratoluene-sulphonic acid and perchloric acid.
Hydroxylation of phenol, carried out under the conditions described, results in a mixture of hydroquinone and pyrocatechol with a predominance of this latter since the hydroquinone/pyrocatechol ratio most frequently varies between 0.3 and 0.7.
A proposal has been made in FR-A 2 266 683 to perfect this process which involves hydroxylation being carried out in the presence of a ketone. There is an improvement in the yield of hydroquinone and pyrocatechol from the reaction. However, all the examples described give a greater quantity of pyrocatechol than of hydroquinone.
The known processes thus mainly yield pyrocatechol.
It transpires that in order to comply with the fluctuating market demand, it is important to have an industrial process which allows an increase in the production of hydroquinone formed in relation to the amount of pyrocatechol.
In order to provide a solution to this technical problem, FR-A 2 667 598 discloses a process which permits an increase in the amount of hydroquinone formed in relation to the amount of pyrocatechol, and which in its preferred variant allows more hydroquinone to be obtained than pyrocatechol.
This process consists in carrying out hydroxylation of the phenol in the presence of an effective amount of a strong acid, said process being characterised by the fact that the reaction is carried out in the presence of a ketonic compound selected from benzophenone and benzophenones, the hydrogen atoms of the aromatic ring of which can be substituted by an electron-donor group.
In accordance with the process described in FR-A 2 667 598, the presence of the ketonic compound such as selected during hydroxylation of the phenol affects the positional selectivity of the reaction, and hydroquinone/pyrocatechol ratios varying between 1.0 and 1.13 are advantageously obtained.
Continuing the research, the inventors have perfected the invention described in FR-A 2 667 598 and found that even higher hydroquinone/pyrocatechol ratios could be obtained by adding to the reaction mixture a small amount of an organic aprotic solvent with certain polarity and basicity properties.